1. Technical Field
The present invention relates to the metrology of optical elements, and in particular, to the metrology of intraocular lenses (IOL).
2. Description of the Related Art
Intraocular lenses have been developed to be implanted in the eye to replace the lens containing cataracts in the eye. During cataract surgery, the capsular bag is cleared of all remnants of the cloudy or damaged biological lens, making space for the insertion of an IOL. In order to properly assess the manufacturing process of intraocular lenses (IOL), the lenses must be measured in a solution which simulates the natural conditions found in the human eye. Solutions employed to measure the IOL's are diverse, from surgical saline to simple deionized water. To conserve solution, control the risk of contamination, and control cost, a minimum amount of solution is used in each measurement. The resulting measurement vessel, known as a cuvette, holds the IOL in its measurement solution in a confined space with limited accessibility.
Toric lens implants are commonly implanted when a patient has significant corneal astigmatism. An integral part of the procedure is to rotate the IOL into the correct position to correct the steep cornea at that meridian. A toric lens has two different optical powers aligned along two meridians on the face of the lens. The meridians are called the steep and shallow meridians and they are perpendicular to each other. IOL manufacturers add fiducial or alignment marks to the IOL, which indicate the expected meridian axis of least power (shallow meridian, also known as the shallow meridian cylinder axis).
When performing optical power and aberration measurement of an IOL, it is desirable to have its cylinder axis oriented in the proper relationship to the measurement system's angular coordinate axis. Often it is advantageous to physically rotate the IOL inside a cuvette during measurement of optical power. Alternatively reorienting the measurement system's angular coordinate axis can be done in software, but at a loss in accuracy.
A wavefront sensor is a device for measuring the optical aberrations of an optical wavefront. This is accomplished by measuring the irradiance and phase distribution of the light beam at a particular plane in space. Although there are a variety of wavefront sensing technologies, including lateral shearing interferometers, curvature sensors, pyramid wavefront sensors, Focault knife-edge test, Ronchi test, and Shack-Hartmann Wavefront Sensor (SHWFS), the SHWFS has been the most frequently employed, since it is capable of measuring both irradiance and phase distributions in a single frame of data.
U.S. Pat. No. 5,936,720 by Daniel R. Neal et al. entitled “Beam Characterization By Wavefront Sensor” issued on Aug. 10, 1999, and U.S. Pat. No. 6,130,419 by Daniel R. Neal entitled “Fixed Mount Wavefront Sensor” issued on Oct. 10, 2000 describe the basics principles of operations of a wavefront sensor employing a two dimensional Shack-Hartmann lenslet array; the disclosures of these patents are incorporated herein by reference. Further details on the use of Shack-Hartmann wavefront sensors in optical metrology may be found in “Application of Shack-Hartmann wavefront sensing technology to transmissive optic metrology” by R. R. Rammage et al., Proc. SPIE Vol. 4779, Advanced Characterization Techniques for Optical, Semiconductor, and Data Storage Components, pp. 161-172, (2002).
U.S. Pat. No. 7,583,389 by Daniel R. Neal et al. entitled “Geometric Measurement System and Method of Measuring a Geometric Characteristic of an Object” issued on Sep. 1, 2009, describes a white light interferometer to measure surface curvature and or thickness of an object. This patent discloses the requirement of tilting of the object with respect to the interferometer apparatus and measuring at a variety of tilt angles in order to characterize a single surface of the object. The disclosure of this patent is incorporated herein by reference.
U.S. Pat. No. 7,623,251 by Daniel R. Neal et al. entitled “Geometric Measurement System And Method Of Measuring A Geometric Characteristic Of An Object” issued on Nov. 24, 2009 describes the use of wavefront sensing to measure surface curvature of an object on one or more surfaces. The measurement requires moving the object relative to the measurement apparatus and measuring at a variety of positions and/or angles in order to characterize the curvature of the one or more surfaces. The disclosure of this patent is incorporated herein by reference.
FIG. 1 depicts a plan view of a toric IOL 22. The optical portion of the IOL 22 lies within the outer diameter 37 of the IOL 22. The haptics 31 function as spacers to center the IOL 22 into the capsular bag of the human eye. The outer diameter 37 of the IOL is large enough to form a good image onto the retina, but too small to fill the capsular bag. The haptics 31 take up the empty space in the capsular bag, thereby centering the IOL 22 into the capsular bag and holding the capsular bag open during healing. The eye is stable once healing is complete.
During the manufacturing process of the IOL 22, fiducial marks 28 are formed onto the surface of the IOL 22 and co-aligned with the expected direction of the shallow meridian cylinder axis 26. The straight line drawn between the fiducial marks 28 shown in FIG. 1 is called the marked shallow meridian (cylinder) axis 26. This line indicates to the surgeon where the shallow meridian cylinder axis of the IOL 22 is expected to be located. The shallow meridian axis 26 defines the line having the lowest optical power of the IOL 22. The expected steep meridian axis 27 is perpendicular to the marked shallow meridian axis 26 as shown in FIG. 1. The intersection of the steep meridian axis 27 and the shallow meridian axis 26 at the surface of the intraocular lens defines the location of the optical axis 29 of the IOL 22 which is orthogonal to both meridian axes.
Depicted in FIG. 2 is a possible manufacturing defect in a toric IOL 22 in which there is a misalignment of the fiducial marks 28 with respect the true location of the shallow meridian axis of IOL 22 represented by dashed line 42. The angular misalignment between the marked shallow meridian axis 26 and the true shallow meridian cylinder axis 42 is represented by misalignment angle 39 in FIG. 2.
During cataract surgery, in a case where the cataract patient has a natural astigmatic cornea, the cataract surgeon implants an IOL 22 with a prescribed amount of cylinder, which neutralizes the corneal astigmatism. The surgeon rotates the IOL 22 taking great care aligning the fiducial marks, 28, with the cylinder axis of the of the patient's cornea, which has been marked in a pre-surgery process. The cylinder added to the IOL corrects the patient's defective cornea. The lower power meridian of the IOL 22 is aligned to the high power of the patient's cornea to balance and even out the patient's corneal astigmatism. Any source of misalignment between the patient's corneal steep meridional cylinder axis and the IOUs shallow meridian axis 42 decreases the effectiveness of the IOL cylinder correction and results in a less than optimal visual outcome for the patient. The surgeon solely relies upon the fiducial marks 28 during surgery and cannot compensate for inherent IOL cylinder axis alignment errors. Therefore it is critical to measure, understand and minimize alignment errors between the fiducial marks and the true location of the shallow meridional cylinder axis 42 of the IOL 22 during manufacturing and prior to surgery. In effect, the misalignment angle 39 should be measured during the manufacturing of the IOL 22 to ensure its contribution to a patient's visual outcome is negligible.
The disclosures of these patents notwithstanding, there remains an unmet need for determining the orientation of fiducial marks placed on a toric intraocular lens with respect to the true location of the shallow meridional axis of the IOL so that the ophthalmic surgeon can properly orient the IOL during surgery before implanting in a patients eye. There is also a need for an apparatus and method that enables the measurement of the transmitted wavefront of a non-axially symmetric lens or other optical element at the proper rotation such that the measurement axis of the wavefront sensor is co-aligned with an optical meridian axis of the optical element under test. Such a capability will enable improved measurement of the physical dimensions and optical performance parameters of multifocal and toric lenses.